So, this car of mine had been sitting from late November 2020 to March 2021 because it had bad brake lines and got a new car so I didn't have to worry about that issue over the winter. Around late March was when me and my friends got around to fixing the lines, and while we were at it, we changed the spark plugs and plug wires because they looked stock , and broke the nozzle going from the EVAP canister purge solenoid valve to the purge canister. Before we changed the plug wires and the spark plugs, and broke the nozzle, the engine wasn't misfiring. afterwards, when I drove it back home, it started to misfire. It idled fairly fine, but was vibrating just enough to indicate a misfire, and was throwing a P0305 at the time. It sat for a bit longer while I tried to find out what to do. Recently I ended up swapping the EVAP purge solenoid from another Impala, and replaced the tube going from the solenoid to the PCV valve. After swapping the solenoid and the tube, it started running a bit better, but it was still misfiring, and somewhere along the way, the P0305 changed to a P0301. While driving to test how bad the misfire is, whenever I press on the gas, I hear a sucking noise coming from the outside of the vehicle. I'm assuming this noise is the engine trying to pull a vacuum, but can't because there'* another leak somewhere. I do have a replacement breather tube coming in that connects from the back of the engine to the other side of the throttle body, because I noticed that that tube isn't sitting properly in it'* spot.

Is there something else that could be causing this engine to misfire instead of a vacuum leak? Or if it is still a vacuum leak, what other parts of the vacuum system would I have to look into?

How many miles on this car?

What did you set the spark plug gap at?

What is the fuel pressure at idle?

The car has about 232k miles, and I gapped the spark plugs to .06 inches, as per the user manual. As for the fuel pressure at idle, I'm not sure.
But I did manage to fix the misfire. Earlier today it started throwing a P300 code, and I decided to swap the EGR from another engine I have lying around to this one, and replacing the gasket in the process.

Great news! Thanks for letting us know what happened!